For various purposes fuel and air or other compounds are brought to reaction to create free energy in the form of heat. This is usually done with the help of burners or combustion chambers for the combustion part and heat exchangers for the exploitation of the thus gained thermal energy. As an example: in many power stations fuel is burned and hot water or steam is produced from the thermal energy with the help of heat exchangers. The whole system often called “boiler”. This steam is then used to drive a turbine in order to produce electricity. An increase in efficiency of such burners and/or boilers—for example for power stations—would lead to a decrease of fuel consumption without decreasing the power-output. An increase of burner-efficiency and/or heat exchanger efficiency or boiler efficiency would lead to an increase of the total efficiency or so called “system efficiency” of such a power station, would save costs, and would decrease the amount of carbon dioxide and excess heat that is created. An increase of burner-efficiency, heat exchanger efficiency and/or boiler efficiency would also allow the use of fuels or compounds with—compared to usual fuel—low energetic value (often incorrectly referred to as ‘low calorific value’) and result in the same efficiency as high energy content fuels; thus allowing the use of otherwise waste products as fuels.
There are several physical effects that are utilised in the present invention. These effects are then used in certain combinations to achieve the desired result. By explaining these physical effects first, it is much easier to understand this invention. These effects are briefly described below, independently from each other:
Propagation Speed and Speed of Expansion
When two compounds—for example a fuel and oxygen inside air—chemically react the reaction between these two compounds has a certain specific propagation speed. Best known is the specific propagation speed of octane with oxygen in the form of air. It is known as Octane Number 100, and used as a comparison for other similar propagation speeds. The octane number system for gasoline for cars is based on this speed and therefore it is well known from daily use at gas stations. By increasing the pressure of the compounds of the reaction, the propagation speed increases and thus the time to complete the reaction decreases. The propagation speed increases exponentially over the increase of the pressure. In this regard it is the pressure of the compounds at the reaction that is important, not the feeding pressure that has no direct influence on the propagation speed. When the pressure is increased and the reaction time is accordingly decreased, the same amount of energy is released in a much shorter time. If the reaction of two compounds takes—as an example—the time of 0.1 seconds, then the energy is released within this time and accordingly the volume of the compounds or gases created increases in a certain specific time that is related to the specific pressure and compounds. The time that is needed to expand is also specific for each mixture and pressure and will remain the same as often as the reaction takes place with the same parameters of pressure and amount or masses of the reacting compounds. The same compounds will always—under the same pressure and with the same amounts—react within the same time. In the example taken above: 0.1 sec. (Exceptions to this rule are mixtures with high amounts of non-reacting compounds.)
If the volume of a mixture of a fuel gas and air is expanding during a reaction—just as an example from 10 cm3 to 1,000 cm3—in their specific reaction time that also depends on the specific heat capacities, their density, etc. then the gases will expand in a much shorter time when the pressure of the compounds in the reaction is increased. Thus the speed with which the resulting gases—that are formed in a reaction of the compounds—expands during and after the reaction is indirectly proportional to the reaction time and thus also directly proportional to the pressure of the compounds during the reaction.
Increase of pressure will lead to increase of speed of the expanding gases that are formed during and after a reaction of the compounds. If the pressure of the compounds is high enough, the compounds will react so fast that they explode or detonate. The definition of “detonation” is more common than the definition for “explosion”. Both—explosion and detonation—refer to a reaction with a speed of the expanding products of the reaction above the speed of sound.
Flame Front Propagation
Usually in a combustion process before they react compounds stream towards the point where they react. This point can be seen as the beginning of the flame. If the compounds are streaming at the same speed (in meters per second) towards the beginning of the flame, then after the reaction away from the reaction point, it looks like the flame is standing still at a certain point to the human eye. In reality, there is a constant flow or movement of the compounds in one direction and the flame front in the opposite direction.
Each mixture of compounds has for each pressure of the mixture its own specific flame speed. If the flame is moving towards the compounds, it is called positive and if the flame front is moving away from the point where the compounds are fed, it is called negative. A negative movement of the flame—usually due to an increase of flow speed of the compounds—leads to a break-up of the flame.
Flame Front Propagation and Reaction Speed of Mixtures
Increasing the pressure of compounds that are able to and are supposed to react increases the reaction speed. With it the speed of flame propagation also increases. As an example a mixture of methane and oxygen increases its reaction speed with pressure. This increase of reaction speed is exponential to the pressure of the reaction. If however the compounds of a reaction are mixed with other compounds that do not or cannot participate in the reaction or form another reaction, then the flame propagation speed will actually decrease. If, considering the example of a chemical reaction between methane and oxygen, there are other compounds—for example the methane is part of a gas mixture of 50 weight percent carbon dioxide and the oxygen is part of natural air, just around 23.151 weight percent—then only around 25 weight percent of the material that forms the total amount of material going to the reaction is able to participate in this reaction. The other compounds are actually hindering the chemical reaction because they are physically in the way between oxygen molecules and methane molecules, preventing them from reaching each other and reacting. By increasing the pressure of the compounds this effect increases and the flame propagation speed decreases. It is also clear that by increasing the pressure of the compounds the density of the buffer material increases and thus becomes less permeable to the compounds that can react chemically. This effect can be compared to fire-protection doors in big buildings that slow down the propagation of a fire, or even hinder it to spread further. Such fire-protection-doors are usually classified according to the amount of time they can delay the spreading of a fire.
Changing Behaviour of Expanding Gases According to their Speed
The terms: expansion, deflagration, explosion and detonation all relate to the behaviour of reacting and thus expanding gases that are usually formed by a chemical or physical reaction of compounds, relative to the speed with which they are expanding. With increasing reaction and expansion speed, the way in which gases expand changes. At relative low subsonic speeds gases expand evenly. Gases that are formed through an explosion or detonation have a different distribution of density In the latter cases, a thin spherical or partially spherical outside layer of the expanding volume—usually referred to as the “shock-wave” or “blast-wave”—has a much higher density than the gases in front of it and especially those behind, as measured relative to the starting point of the explosion or detonation. The gases behind the “shock-wave” are commonly assumed to have a low pressure or vacuum. The pressure of a wave-front from an explosion or detonation on a Wall (when the spherical or partially spherical wave front hits the wall and the mass of the wave front goes through a negative acceleration) is much higher than the average pressure of these compounds at the point of time when the reaction starts. In other words: the amount of energy that is in the spherically or partially spherically expanding gases is not evenly distributed throughout these gases but is highest at the outside, at the “shock-wave-front”.
Gas Friction or Fluid Resistance and its Increase over Speed
Friction, also called ‘fluid resistance’, is created when pressurised gases flow through pipes or systems similar to pipes. This gas friction or fluid resistance increases with exponentially pressure and with speed. This can best be understood as the mechanical collision of molecules or atoms of the gas or fluid passing through the pipe with molecules or atoms of the pipe. The colliding molecules or atoms are thrown back into the stream and create a flow pattern as a result of being thrown back by the collision that disturbs the free flow until they create a blockage. This can be also compared with a multilane highway and cars travelling on this highway in one direction. If a few cars sporadically collide at the outside lanes with obstacles, they will be catapulted back onto the highway and will cause more collisions with following cars. If the speed is increased, the damage is considerably larger. It is clear that a car that smashes into an obstacle at higher speed will therefore be catapulted further back into the stream. Also, when the density—the flow—or number of cars is increased with more cars behind each other, the flow will be more disrupted by such collisions at the borders. Finally, if the highway gets narrower, the interference with free flow will also increase when collisions at the sides of the highway occur. At a certain speed, that is different for each specific gas stream—according to its composition, temperature, and pressure—the gas friction or fluid resistance is so high, that no more gases can pass through the pipe. The gases are then blocked from flowing by gas friction or fluid resistance.
Boundary Layers
At a solid surface, boundary layers can and will form. For example if a stream of gas flows over a solid surface than the molecules of the gas that are closest to the solid surface will change their path of flow—due to the surface structure of the solid material. Also if a hot gas is streaming over a relatively colder solid surface—no matter whether turbulent or laminar—the gas will transfer a part of its thermal energy to the solid surface and therefore change its properties, primarily its temperature and secondarily its density and therefore volume, thus creating a layer with different flow properties—also referred to as a “boundary layer” between the solid surface and the main part of the gas flow. If gases have to transfer heat energy into solids in heat exchangers, these boundary layers create—mostly unwanted—buffers between the solid wall and the main part of the gas stream, thus significantly decrease the efficiency of heat transfer.
Also, if a stream of hot or warm gases is flowing in a turbulent or laminar manner over a relatively cold surface of the heat exchanger the exchange of energy cools the hot gases and thus also changes their flow pattern. It also creates the natural effect that gases that are on the other side of the gases that just exchanged energy with the wall of the heat exchanger are now hotter and therefore again heat up the gases that just exchanged their heat energy with the heat exchanger wall. Thus, the process of heat exchanging by flowing hot gases over relative colder walls of the heat exchanger creates a pattern that leads to a decrease of the energy transfer. The effect of exchange of energy between colder and warmer gases leads to a decrease of efficiency of heat exchange with laminar or turbulent gas streams due to the creation of layers with smaller temperature differences to the next layer.
Another important point is the successive decrease of the temperature difference between the hot gases and the solid surface. A stream of hot gases with a nominal temperature measured in the middle of the hot gas stream has a certain temperature difference to the solid surface. The higher this temperature difference, the higher is the possible heat exchange rate. The boundary layer however creates layers of gases that have already exchanged heat with the solid surface and thus act as buffers of lower temperatures—like insulation—between the hot area of the gas stream and the colder solid surface. Thus the temperature difference between the hot gas stream and the colder solid surface cannot be used for the heat exchange, just the much lower temperature difference between the molecules of the boundary layer—that have a lower temperature than the main gas stream—and the solid surface.
To overcome this effect of the boundary layers, often turbulent streaming of the hot gases is used—instead of laminar streaming. The hot gases are streaming turbulently and can therefore exchange and replace layers that are forming at the boundaries to the solid surfaces. However, using turbulent streaming instead of laminar streaming of hot gases leads to the effect that more time is necessary to perform the heat transfer. This means that the surface that is covered by turbulent streaming is bigger than the surface that is covered in the same time by a laminar streaming. Therefore the active surface where the heat exchange takes place has to be bigger than would be necessary if there were no effects like the boundary layer, and thus the heat is spread over a bigger surface. Therefore, as a direct consequence, the available temperature also decreases and the same amount of energy has to heat up a bigger surface. Even though the net heat transfer is more efficient with turbulent streaming than with laminar streaming, in both cases only a part of the heat can be transferred.
Due to these effects as described directly above it is commonly said that the higher the temperature of the gases that are produced in the combustion or incineration, the better the overall efficiency of the system. This is not because the system depends on the primary temperature of the combustion or incineration but only because the effects described above make it impossible to gain a higher amount of the heat with conventional heat exchangers that depend on streaming—whether laminar or turbulent—of hot gases over solid surfaces.
From U.S. Pat. No. 6,555,727 (Michael L. Zettner) a burner concept is known where the compounds are fed under pressure and react “explosion-like” very rapidly. In this concept the flame does not break off, burning in a discontinuous way, but clearly burns continuously. Burners that operate under pressure have to deal with the problem of a flame front breaking off and many mechanisms had been invented to overcome this problem. For example U.S. Pat. No. 5,131,840 (Michael L. Zettner) presents a method for preventing the break off of the flame front.
So called “Pulse Detonation Engines” have been known for more than 70 years and a few have even been built and tested. The most famous was the “Argus AS 109-014 pulse jet engine” that was used as the engine for the German “V1 flying bomb”. It had mechanical valves or shutters to prevent the backwards movement of the wave front of the explosion or detonation and reached around 50 Hz as frequency. It neither utilised the heat nor the friction of the pulsed explosions or detonations. A more modern form is the heavily modified “Rutan Long” type EZ as well as several experiments in connection with the DARPA Falcon project in the military industry in the USA. Also in these cases only frequencies of 200 Hz have been reached and mechanical means are used to control the frequency of the detonations.
From publications like Shchelkin “Gas Dynamics of Combustion” from 1965 or more contemporary publications that refer to Shchelkin e.g.: University of Texas Arlington Panicker, Philip (2007) “Experimental Investigation of DDT Enhancements by Shchelkin Spirals”, and University of Texas Arlington Lu, F.K.; Meyers J.M.; Wilson, D.R. (2007), “Experimental study of a pulse detonation rocket with Shchelkin Spiral”, it is known that there are means to increase gas friction or drag in pulse detonation engines in order to decrease or minimise the back-flow of the wave front. However, these means are not designed to stop the wave front completely and they are also not means to control the pulsing of the detonation. The publication of Philip Panicker shows on slide 15/27 the “3-way Rotary Valve” for feeding and also on the same page above the rotary valve spring operated back-flow valves. The Shchelkin Spiral disintegrates after a very short time due to the fact that it has to stand within the backflow of the wave front of the explosion or detonation and therefore receives extreme negative acceleration and heat from the wave front of the explosion or detonation. These extreme forces destroy the spiral after a few seconds of operation according to the findings of Philip Panicker and the photos he has published in the aforementioned publication. The Shchelkin Spiral sits between the point of ignition and the outlet and thus results in the blocking of a wave front of an explosion or detonation. The task of the Shchelkin Spiral is in no case to create a pulsing effect or to prevent backflow of wave fronts from explosions or detonations.
It is a purpose of the present invention to provide a burner system that allows ‘quasi continuous burning’ of all kinds of fuels at very high temperatures by using controlled continuous pulsing explosions or detonations to create pressure waves that can be easily utilised for increasing heat exchanger efficiency.
It is another purpose of the present invention to provide a burner system that depends on a break off of the flame and uses the effects of the explosion or detonation that blows out the flame for increased heat transfer into the heat exchanger wall.
It is another purpose of the present invention to provide a burner system that works without any moving parts and or valves.
Further purposes and advantages of this invention will appear as the description proceeds.